Calculate CU on a lamp-lumen basis or utilance on a luminaire-lumen basis. Also solve emitted or workplane flux, average illuminance, luminaire count, source lumens per luminaire, or room cavity ratio.

Match the flux basis: CU uses lamp lumens; utilance uses luminaire lumens. Use the matching manufacturer factor.

Coefficient of Utilization Formula

The following equation is used to calculate the Coefficient of Utilization.

CU = LRP / LU
  • Where CU is the coefficient of utilization
  • LRP is the luminous flux reaching the desired plane (lumens)
  • LU is total lamp luminous flux before fixture optical losses (lumens). If the denominator is luminaire output instead, the ratio is utilance, not lamp-based CU.

For CU, divide workplane flux by total lamp flux. For utilance, divide it by luminaire output. Select the matching basis in the widget and use the same basis for manufacturer factors and source lumens. See the IES definition of CU and utilance.

What is a Coefficient of Utilization?

Definition:

CU describes workplane flux relative to lamp flux; utilance uses luminaire output. The lumen method estimates average maintained illuminance as E = N × source lumens × matching utilization factor × LLF / area. Solve for N by rearranging and rounding upward, or for source lumens by rearranging without whole-lumen rounding. Convert footcandles to lux once (1 fc = 1/0.09290304 lux) and ft² to m² once (1 ft² = 0.09290304 m²). These averages do not verify uniformity, glare, spacing or compliance.

How to Calculate Coefficient of Utilization?

Example Problem:

The following example calculates lamp-based CU. Generic reflectance presets cannot supply a reliable factor. For a rectangular room, RCR = 5h(1/L + 1/W), where h is the distance from the workplane to the luminaire plane. Use RCR and measured reflectance conditions with the specific manufacturer table; the widget does not infer CU from geometry or luminaire type.

First, determine total lamp flux before fixture optical losses. In this CU example, the lamps produce 5000 lumens.

Next, determine the luminous flux that reaches the desired plane. In this case, this value is 2000 lumens.

Finally, calculate the coefficient of utilization using the formula above:

CU = LRP / LU

CU = 2000 / 5000

CU = .4